1. Field of the Invention
This invention relates to systems and methods for improving the quality of video images generated by a forward-looking infrared (FLIR) sensor array. These systems and methods combine the means and method steps of scene-restored systems and methods, but determine the responsivity or conversion gain for each sensor in the system by an autocorrelation/cross-correlation method, and without scanning a constant temperature source.
2. Description of Related Art
In a thermal imaging sensor, it is desirable that object points or pixels of significantly different radiance generate image pixels of significantly different luminance-significant as compared to the background noise level. A corollary requirement is that for good image quality, object pixels of equal radiance should generate image pixels of equal luminance. The ground-sky horizon provides a severe test of the ability of a parallel scan, multi-element sensor to meet these two requirements.
The earliest parallel-scan, thermal imaging sensors had AC- coupled signal processors. AC coupling removes the DC component of the outputs, making the average output of all channels equal, independent of the scene. The average output of each channel, however, represents the average radiance along its object space scan line, and these are not usually equal. If, for example, the scene consists of featureless sky and ground areas separated by the horizon, the average radiance represented by each output is: EQU R(ave)=[A(sky)*R(sky)+A(gnd)*R(gnd)]
where A(sky) and A(gnd) are individual weighting factors (whose sum is unity) for each channel, equal to the fractions of the total field-of-view occupied by the sky and ground respectively. If the earth and sky are within the linear range of the signal processor, then the sky and ground signals [S(sky) and S(gnd)]are: ##EQU1## where G(conv) is the conversion gain (volts or lumens per watt).
Scan lines lying entirely in the sky or ground result in one of the weighting factors being zero. The average outputs for those channels, though equal, then represent two different radiant inputs and violate the minimum requirement. If an AC coupled sensor used for "nap of the earth flight" is oriented with its scan lines parallel to the horizon, the horizon disappears, presenting an obvious problem. When the scan raster is skewed relative to the horizon a different problem arises, and the sensor fails to meet the second requirement. In this case the weighting factors for those channels whose scan lines cross the horizon can have any value between zero and unity. As shown by the equations, the sky and ground signals can then take on all values between zero and the full sky-to-ground difference.
DC restoration provides a partial solution to this problem by using a thermal reference source (TRS) to provide a common reference radiance for all sensor elements. The response of each element to this common input is stored and subtracted from each pixel signal [S(pxl)]. The use of a TRS requires modifying the previous equations, and the average radiance becomes: ##EQU2##
Since each AC coupled pixel signal (including the TRS)
is: EQU S(pxl)=G(conv)*[R(pxl)-R(ave)]
the corresponding DC restored signal [X(pxl)] is: ##EQU3##
As shown in Equation 1, the weighting factors do not appear in the new output equation. The DC Restored output signals are strictly proportional to the difference between the image pixel radiance and the TRS radiance and all sky and all ground image pixels of the example appear with equal but different luminances.
A common reference source for all elements also solves a related problem with DC coupled sensors, whose individual outputs are the sum of a signal-independent random offset, and a term proportional to the product of the object pixel radiance and each element's conversion gain. In this case the process may be more properly called Level Equalization, rather than DC Restoration. It eliminates the random variation in raster line luminance which would appear in the image of an uncorrected, DC coupled sensor viewing a uniform temperature scene whose radiance is equal to that of the TRS. Image areas whose radiance were significantly different from the TRS would, however, for both the DC Restored, AC coupled sensor, and the Level Equalized, DC coupled sensor, exhibit random variations in raster line luminance due to inter-element conversion gain variations.
The previous equations implied that the conversion gain [G(conv)] was a constant for all channels. In general, significant inter-channel variations exist, causing output variations which are proportional to the product of the interchannel gain errors and the TRS to scene differential radiance. The effects are most apparent in images combining large areas and large temperature differences. The previous example of the horizon separating uniform-temperature sky and ground areas, but now viewed by an orthogonal raster, provides a good example. Errors of this type must be removed by equalizing the conversion gains, a process commonly called Responsivity Equalization (RE). RE involves multiplying each channel's output by the ratio of a constant (the average responsivity for all the sensor channels, for example) to the individual channel's responsivity. Referring to the previous equations, each conversion gain then becomes equal to, in our example, that of the average responsivity channel.
First generation direct view sensors (such as the common modules) used potentiometers to trim the gain of the parallel channels for equal responsivities. Later first generation sensors, with parallel outputs multiplexed to a single serial output, stored equalization data in read only memory and applied it to the multiplexed video. Adding a second TRS makes automatic Responsivity Equalization possible by continuously adjusting each channel's gain to produce the same signal difference for the two sources. This, however, requires compromising the other sensor requirements or increasing optomechanical complexity. These latter considerations led to the use of Scene-Based RE (SBRE) in some sensors. Simple SBRE acts like an independent Automatic Gain Control (AGC) for each channel, producing equal outputs for all channels. The potential problem with this approach is that the signals are the product of the responsivities and the radiant input, and equal outputs do not guarantee equal responsivities. The assumption is usually made that over a long time interval, all channel inputs are statistically equal, and if the gains converge slowly, the correct results will be obtained.
This invention provides SBRE methods and systems which include a test to determine if differences in the outputs of adjacent channels result from different inputs or from different conversion gains. This makes it possible for the conversion gains to converge rapidly with a high degree of confidence that the conversion gains, and not the output signals, are being equalized.